An inertial measurement system comprising: a first, roll gyro with an axis oriented substantially parallel to the spin axis of the projectile; a second gyro and a third gyro with axes arranged with respect to the roll gyro; a controller, arranged to: compute a current projectile attitude from the outputs of the first, second and third gyros; operate a Kalman filter that receives a plurality of measurement inputs including at least roll angle, pitch angle and yaw angle and that outputs at least a roll angle error; initialise the Kalman filter with a roll angle error uncertainty representative of a substantially unknown roll angle; generate at least one pseudo-measurement from stored expected flight data; provide said pseudo-measurement(s) to the corresponding measurement input of the Kalman filter; and apply the roll angle error from the Kalman filter as a correction to the roll angle.
Legal claims defining the scope of protection, as filed with the USPTO.
1. An inertial measurement system for a spinning projectile comprising: a first, roll gyro with an axis oriented substantially parallel to the spin axis of the projectile; a second gyro and a third gyro with axes arranged with respect to the roll gyro such that they define a three dimensional coordinate system; a controller, arranged to: compute a current projectile attitude from the outputs of the first, second and third gyros, the computed attitude comprising a roll angle, a pitch angle and a yaw angle; operate a Kalman filter that receives a plurality of measurement inputs including at least the roll angle, pitch angle and yaw angle and that outputs at least a roll angle error; initialise the Kalman filter with a roll angle error uncertainty representative of a substantially unknown roll angle; generate at least one pseudo-measurement from stored expected flight data, the or each pseudo-measurement corresponding to an expected measurement input of the Kalman filter; provide said pseudo-measurement(s) to the corresponding measurement input of the Kalman filter; and apply the roll angle error from the Kalman filter as a correction to the roll angle; wherein the Kalman filter is arranged to calculate the roll angle error as a function of the pseudo-measurement(s).
2. An inertial measurement system as claimed in claim 1 , wherein the spinning projectile is arranged to be powered off at launch and to power up after launch.
3. An inertial measurement system as claimed in claim 1 , wherein the roll angle uncertainty is initialised as at least 90 degrees, preferably at least 135 degrees, more preferably at least 160 degrees, and most preferably 180 degrees.
4. An inertial measurement system as claimed in claim 1 , wherein the at least one pseudo-measurement comprises a pitch angle pseudo-measurement and a yaw angle pseudo-measurement generated from stored expected pitch angle data and stored expected yaw angle data respectively.
5. An inertial measurement system as claimed in claim 1 , wherein the at least one pseudo-measurement comprises a GPS position pseudo-measurement generated from stored expected position data.
6. An inertial measurement system as claimed in claim 1 , wherein the at least one pseudo-measurement comprises a GPS velocity pseudo-measurement generated from stored expected velocity data.
7. An inertial measurement system as claimed in claim 1 , wherein the at least one pseudo-measurement comprises a velocity pseudo-measurement generated from a combination of measured platform speed with stored expected pitch angle and stored expected yaw angle.
8. An inertial measurement system as claimed in claim 4 , wherein the difference between the pitch angle and the pitch angle pseudo-measurement is treated as a component of a Kalman filter innovation vector, and wherein the difference between the yaw angle and the yaw angle pseudo-measurement is treated as another component of a Kalman filter innovation vector.
9. An inertial measurement system as claimed in claim 1 , wherein the controller is arranged to stop providing pseudo-measurements to the Kalman filter in a subsequent phase of flight.
10. An inertial measurement system as claimed in claim 1 , wherein the controller is arranged to apply the roll angle correction to an attitude integration unit.
11. An inertial measurement system as claimed in claim 1 , wherein the controller is arranged to calculate and apply a roll rate scale factor correction directly to the roll gyro output.
12. An inertial measurement system as claimed in claim 1 , wherein the expected trajectory is planar ballistic flight or near-planar ballistic flight.
13. An inertial measurement system as claimed in claim 4 , wherein the expected pitch angle and expected yaw angle or the expected position data or the expected velocity data as a function of flight time are taken from a pre-computed flight trajectory which may be non-planar.
14. An inertial measurement system as claimed in claim 1 , wherein the expected trajectory is recalculated after an initial flight phase based on current estimated pitch and yaw angles from the inertial measurement system.
15. A method of correcting roll angle in an inertial measurement system for a spinning projectile, comprising: computing a current projectile attitude from the outputs of first, second and third gyros, the computed attitude comprising a roll angle, a pitch angle and a yaw angle; operating a Kalman filter that receives as inputs a plurality of measurements including at least the roll angle, pitch angle and yaw angle and that outputs at least a roll angle error; initialising the Kalman filter with a roll angle error uncertainty representative of a substantially unknown roll angle; generating at least one pseudo-measurement from stored expected flight data, the or each pseudo-measurement corresponding to an expected measurement input of the Kalman filter; providing said pseudo-measurement(s) to the corresponding measurement input of the Kalman filter; and applying the roll angle error from the Kalman filter as a correction to the roll angle; wherein the Kalman filter is arranged to calculate the roll angle error as a function of the pseudo-measurement(s).
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April 27, 2018
February 25, 2020
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